1. Field of Invention:
This invention relates generally to the field of lubricants, and more particularly to an enhanced grease having two major components--the first being a hybrid lubricant constituted by a stable suspension of sub-micronic PTFE particles in an oil carrier; the second component acting as a gelling agent for the first component and being in the form of a standard grease composed of a thickener dispersed in a lubricating oil.
The search for effective liquid and semi-solid lubricants probably got under way with the invention of the wheel and is today still in progress, for lubricants currently available fall well short of ideal. A milestone in this continuing search is the development in about 1400 B.C. by the ancient Egyptians of an axle grease for chariot wheels made up of animal fat and a calcium soap thickener. Another notable event in the history of lubrication is the use by the Chinese in 780 A.D. of a mixture of vegetable oil and calcined lead.
Even the most carefully finished metal surfaces have minute projections and depressions therein which introduce resistance when one surface shifts relative to another. The application of a fluid lubricant to these surfaces reduces friction by interposing a film of oil therebetween, this being known as hydrodynamic lubrication. In a bearing, for example, the rotation of the journal causes oil to be drawn between it and the bearing so that the two metal surfaces are then separated by a very thin oil film. The degree of bearing friction depends on the viscosity of the oil, the speed of rotation and the load on the journal.
Should the journal start its rotation after a period of rest, it may not drag enough oil to float the surfaces apart; hence friction would then be considerably greater, the friction being independent of the viscosity of the lubricant and being related only to the load and to the "oiliness" property of the residual lubricant to stick tightly to the metal surfaces. This condition is referred to as "boundary lubrication," for then the moving parts are separated by a film of only molecular thickness. This may cause serious damage to overheated bearing surfaces.
The two most significant characteristics of a hydrodynamic lubricant are its viscosity and its viscosity index, the latter being the relationship between viscosity and temperature. The higher the index, the less viscosity will change with temperature. Fluid lubricants act not only to reduce friction, but also to remove heat developed within the machinery and as a protection against corrosion.
Though fluid film separation of rubbing surfaces is the most desirable objective of lubrication, it is often unobtainable in practice. Thus bearings built for full fluid lubrication during most of their operating phases actually experience solid-to-solid contact when starting and stopping. In some instances, the use of a solid lubricant alone or in combination with a liquid lubricant is indicated.
Typical solid lubricants are soft metals such as lead, the layer lattice crystals such as graphite and molybdenum disulphide, as well as the crystalline polymers such as Teflon (polytetrafluoroethylene or PTFE).
Under severe operating conditions, hydrodynamic or fluid lubrication is inadequate to minimize friction and wear; for fluid film separation of the rubbing surfaces is not possible throughout all phases of operation. Hence, the better lubricant for an engine or other mechanism having moving parts is one which combines hydrodynamic with solid lubrication. In this way, when adequate separation exists between the rubbing surfaces, a protective fluid film is interposed therebetween; and when the surfaces are in physical contact with each other, friction therebetween is minimized by layers of solid lubricant bonded to the surfaces.
In theory, one can best approach this ideal by interposing the rubbing parts of engines with solid lubricant layers which are integrally bonded thereto, concurrent use being made of a lubricating oil which functions not only to provide hydrodynamic lubrication but also to cool the rubbing parts. In addition, the oil may carry synthetic organic chemicals to perform other functions to counteract wear and prevent corrosion.
The practical difficulty with attaining this ideal is that the parts coated with solid lubricants, such as a PTFE layer, are very expensive and therefore add considerably to the overall cost of the engine. Moreover, in PTFE-coated parts which operate under rigorous conditions, the solid lubricant layers bonded thereto have a relatively short working life, so that it is not long before the only lubricant which remains effective in the engine is the fluid lubricant.
In order to provide a lubricating action which is both solid and fluid, my prior U.S. Pat. No. 3,933,656 discloses a modified oil lubricant which is suitable for applications which call for effective lubrication throughout all phases of operation. This modified lubricant is constituted by major amounts of a conventional lubricating oil intermingled with minor amounts of polytetrafluoroethylene particles in the sub-micronic range in combination with neutralizing agents which stabilize the dispersion to prevent agglomeration and coagulation of the particles.
The present invention is concerned exclusively with greases for use in those applications in which a liquid lubricant is inappropriate. Originally, greases were thick crude oils or semi-solid fats. In the modern age, greases are now specially prepared solid or semi-solid dispersions of organic or inorganic thickeners in lubricating oils to which silicones and other ingredients are added in minor amounts to impart special properties to the grease.
A grease rather than a liquid lubricant is used where high bearing pressures are encountered, or where oil drip from bearings is interdicted. Grease is also necessary where the motion of the contacting surfaces is discontinuous, making it difficult to maintain a separating film therebetween. Grease is generally employed where the parts to be lubricated are difficult to get at, or where a danger exists that a dripping liquid lubricant will contaminate the product being produced by the lubricated machine. An acceptable lubricating grease will flow into bearings by the application of pressure, and it will remain in contact with the moving surfaces and not leak out under centrifugal action or by reason of gravity flow.
Thickeners for grease include soap and non-soap types. Soap thickeners are constituted by compounds having cations of Al, Ba, Na, Pb, Li or Sr, these being formed in situ by reacting a metal base with fatty acids. Those non-soap inorganic thickeners which enjoy the widest application include colloidal thickeners or modified clays such as bentonites, which are rendered organophilic and water repellent. A description of metallic soaps usable as thickeners is contained in the Witco Metallic Stearates handbook published by Witco Chemical--Organics Division (1970).
The value of a lubricating grease over a broad temperature range depends both on its apparent viscosity at low temperatures and its resistance to deterioration at elevated temperatures. The apparent viscosity is a function of the pour point and viscosity index of the fluid employed in the grease. The pour point of an oil is the lowest temperature at which an oil will flow when maintained under quiescent conditions. Lack of stability at high temperatures can arise from collapse of the structure or deterioration of one or more of the grease ingredients. Thus a grease which fails to adequately reduce friction will cause the lubricated bearing surfaces to overheat; and this, in turn, will cause the breakdown of the grease and destruction of the bearings.
When circulating oils act to lubricate cams, tappets, timing gears and piston rings, or when the parts to be lubricated operate under boundary conditions, anti-wear additives are necessary. Use is often made for this purpose of zinc dithiophosphate which also acts as a corrosion inhibitor. It is the chemical characteristic of this substance which affords wear resistance. Anti-wear agents function through a chemical polishing action which can take place at relatively low temperatures. However, anti-wear additives do not increase the lubricity of the oil, and in fact may introduce friction.
In high speed machinery in which the bearings are lubricated by conventional greases having anti-wear additives, the greases do not usually reduce friction to a degree preventing excessive heating of the bearings. Consequently, as the grease proceeds to deteriorate at elevated operating temperatures, the metal parts become scored and failure is experienced after a relatively short period.
2. Prior Art
U.S. Pat. No. 3,159,557 to Ambrose entitled "Grease Containing Polytetrafluoroethylene" discloses a grease having PTFE particles dispersed therein to create an extreme high pressure lubricant. In compounding this grease, a thickening soap is first prepared, the soap then being admixed with mineral or synthetic oil to form a grease, after which the PTFE particles are added to the grease composition.
The problem with a grease of the Ambrose type is that when it is put to use in bearings and in other moving parts, and the grease is then rendered fluid to effect hydrodynamic lubrication, the PTFE particles dispersed in the fluid will tend to agglomerate. As noted in my prior U.S. Pat. No. 4,127,491, under high shear and impact, these agglomerates will consolidate into a tough, gummy mass that interferes with lubrication and has deleterious results. The fact that PTFE particles, when added to a lubricant, because of their inherent instability tend to agglomerate, is also noted in the Browning U.S. Pat. No. 3,194,762.
The present invention makes use, as one component thereof, of a stable hybrid lubricant of the type disclosed in my above-identified prior patents in which a colloidal dispersion of PTFE particles in an oil carrier are stabilized to prevent agglomeration of the particles, use being made for this purpose of a neutralizing agent that acts to maintain the particles in suspension regardless of the prevailing working conditions.